Metal oxide varistor with coating that enhances contact adhesion

ABSTRACT

Disclosed is a metal oxide varistor comprising a body portion consisting essentially of a metal oxide and a small percentage of a plurality of preselected additives. The body portion defines two major surface areas and metallic contacts are applied to the two surface areas. Beneath the contacts there is a coating that enhances the adhesion of the contacts. In a preferred embodiment disclosed herein the coating contours the surface of the body and thereby enhances contact adhesion.

United States Patent May 1 Sept. 2, 1975 [5 METAL OXIDE VARISTOR WITH COATING 3,496,512 2/1970 Matsooka .1 338/20 THAT ENHANCES CONTACT ADHESION 3,512,057 8/1970 Hatchen, Jr. 1 17/212 [75] Inventor: John E. May, Skaneateles, N.Y. Pn-"mry Examiner john D- welsh [73] Assignee: General Ele tri Com Attorney, Agent, or FirmRobert J. Mooney; Douglas Syracuse, NY. E. Stoner 22 F'] d: S t. 27, 1973 1 ep 57 ABSTRACT [21] Appl' 401307 Disclosed is a metal oxide varistor comprising a body portion consisting essentially of a metal oxide and a [52] US. Cl. 338/20; 338/21; 338/238; m l per ntage of a plurality of preselected addi- 1 17/212; 1 17/213; 117/217; 338/327 tives. The body portion defines two major surface [51] Int. Cl. HOlc 7/10; HOlc 7/12 areas n m llic on acts re applied to the two sur- [58] Field of Search... 1 17/212, 213, 217; face areas. Beneath the contacts there is a coating that [56] References Cited UNITED STATES PATENTS 3,069,294 12/1962 Davis 117/212 enhances the adhesion of the contacts. In a preferred embodiment disclosed herein the coating contours the surface of the body and thereby enhances contact adhesion.

8 Claims, 2 Drawing Figures PATENTL'U SEP 2 975 METAL OXIDE VARISTOR WITH COATING THAT ENHANCES CONTACT ADHESION BACKGROUND OF THE INVENTION This invention relates to metal oxide varistors and, more particularly, to a coating which, when applied to a varistor body prior to the application of metallic contacts, enhances the adhesion of the contacts to the varistor body.

In general, the current flowing between two spaced points is generally directly proportional to the potential difference between those points. For most known substances, current conduction therethrough is equal to the applied potential difference divided by a constant, which has been defined by Ohms law to be its resistanee. There are, however,-a few substances which exhibit non-linear resistance. Some devices, such as metal oxide varistors, utilize these substances and require resort to the following equation (1) to quantitatively relate current and voltage:

where V is the voltage applied to the device, I is the current flowing through the device, C is a constant and a is an exponent greater than 1. Inasmuch as the value of 01 determines the degree of non-linearity exhibited by the device, it is generally desired that a be relatively high. a is calculated according to the following equation (2):

where V and V are the device voltages at given currents hand 1 respectively.

At very low voltages and very high voltages metal oxide varistors deviate from the characteristics expressed by equation l and approach linear resistance characteristics. However, for a very broad useful voltage range the response of metal oxide varistors is as expressed by equation (1).

The values of C and 01 can be varied over wide ranges by changing the varistor formulation and the manufacturing process. another useful varistor characteristic is the varistor voltage which can be defined as the voltage across the device when a given current is flowing through it. It is common to measure varistor voltage at a current of one milliampere and subsequent reference to varistor voltage shall be for voltage so measured. The foregoing is, of course, well known in the prior art.

Metal oxide varistors are usually manufactured as follows. A plurality of additives is mixed with a powdered metal oxide, commonly zinc oxide. Typically, four to twelve additives are employed, yet together they comprise only a small portion of the end product, for example less than five to ten mole percent. In some instances the additives comprise less than one mole percent. The types and amounts of additives employed vary with the properties sought in the varistor. Copious literature describes metal oxide varistors utilizing various additive combinations. For example. see U.S. Pat. No. 3,663,458. A portion of the metal oxide and additive mixture is then pressed into a body of a desired shape and size. The body is then sintered for an appropriate time at a suitable temperature as is well known in the prior art. Sintering causes the necessary reactions among the additives and the metal oxide and fuses the mixture into a coherent pellet. A passivating eoating is sometimes applied to the sintered body. If a coating is applied, the body with the coating is generally reheated. Next, metallic contacts are applied to the body. The contacts can, for example, be applied by techniques such as the application of a silver paste or by metallic flame spraying.

A problem encountered in metal oxide varistors manufactured by the prior art method has been contact failure. Occasionally a contact will develop a crack or tear near the lead attachment or will peel from the varistor body entirely. Either of these events can, of course, lead to device failure. It is felt that both of the above failure mechanisms are caused by inadequate contact adhesion. Thus, contact adhesion has become a matter of concern to varistor manufacturers.

It is, therefore, an object of this invention to provide a varistor with contacts that exhibit enhanced adhesion to the varistor body.

SUMMARY OF THE INVENTION This invention is characterized by a metal oxide varistor comprising a body portion that is composed essentially of a metal oxide and a small percentage of a plurality of preselected additives. The metal oxide and the additives are mixed and then a portion of the mixture is pressed into a body of a desired shape and size. The pressed body is then sintered to form the varistor body in a manner well known in the prior art. A coating is then applied to the body and it is reheated as described below. Metal contacts are applied and wire leads are attached to the contacts. The device is then ready to be encapsulated.

A feature of the invention is that the coating enhances the contact adhesion. A preferred method of enhancement is to contour the surface of the varistor body. For example, particulate matter can be included in the coating and the step of heating the coating can be carried out, as described below, at a temperature that does not melt the particulate matter.

Inasmuch as metal contacts, when applied by such conventional methods as the application and heating of silver paste or the utilization of a metallic flame spray, closely conform to the contour of the substrate surface, in this case the varistor pellet surface, it will be appreciated that a substantial increase in contact adhesion will be realized by the utilization of a contoured pellet surface.

It is, of course, realized that other methods of contouring the surface of the pellet can be employed. For example, a coating that shrinks as cured can be employed. The shrinking coating will develop cracks that 'create a contoured surface effect.

Inasmuch as coatings are often applied to varistor bodies when manufactured by conventional methods, it will be appreciated that little extra cost will be incurred in the manufacture of the subject varistor if a conventional coating currently in use is modified to provide a surface contouring effect.

Another feature of the coating is that it is free of silver and other monovalent ions that tend to diffuse rapidly into the varistor pellet during the step of heating the coating. It is advantageous to prevent such ion diffusion inasmuch as it causes high leakage current and other effects on the varistor properties that are difficult to predict. Furthermore, the coating can be made free of silicon as disclosed below. It has been found that the inclusion of silicon in a varistor pellet coating increases the varistor voltage of the treated device. Thus, when fabricating low voltage varistors, it is preferable that the coating be free of silicon.

Still another feature of the subject coating is that it is a passivating coating and substantially improves the stability of the devices as discussed below. In order to provide a coating compatible with the varistor body portion, the preselected additives form a major portion of the coating. Thus, compatibility of the coating and the varistor is assured.

The preselected additives can be prereacted to form a reaction product which is ground and then mixed with the metal oxide prior to pellet pressing and sintering. It has been found that such a technique provides a varistor with very desirable electrical properties, high stability and of a very uniform composition. A full dismission of the method for fabricating a varistor by pre reacting the additives is in my co-pending U.S. patent application Ser. No. 401,13] filed concurrently herewith and entitled Low Voltage Varistor and Process for Making. The coating can comprise the reaction product. A carrier can be included, as is disclosed below, to enhance the adhesion of the reaction product to the body portion surface. In order to further assure compatibility, the particulate matter that contours the pellet surface can be grains of the reaction product.

Consequently, it will be appreciated that there has been provided a coating for metal oxide varistors that functions as a passivating coating to improve the stability of the device. The coating is fully compatible with the varistor body portion, and, inasmuch as the coating is free of monovalent ions, does not adversely alter the electrical properties of the varistor. Furthermore, the coating can be free of silicon if a low voltage device is being manufactured. Finally, the coating enhances the adhesion of the contacts to the varistor body portion, thus preventing device failure due to detached, cracked or torn contacts.

DESCRIPTION OF THE DRAWINGS These and other features and objects of the present invention will become more apparent upon a perusal of the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a sectional view of a preferred metal oxide varistor; and

FIG. 2 is a detailed sectional view of a portion of the varistor depicted in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT AND METHOD Referring first to FIG. 1 there is shown a metal oxide varistor that includes a sintered body portion Ill composed essentially of a metal oxide and a plurality of preselected additives. The body 11 can be manufactured by methods well known in the prior art or by an improved technique to be discussed below. Surrounding the body portion is a passivating coating 12 and, overlying the coating on two major surface areas of the body, are two metallic contacts 13 and 14. Two wire leads l5 and 16 are coupled to the contacts 13 and i4,

respectively, by conductive coupiers such as solder l7 and 18.

Referring now to FIG. 2 there is shown a portion of the varistor l0 shown in FiG. i. Specifically, a corner of the body portion 11 is shown in HQ. 2. In the enlarged FIG. 2, the passivating coating 12 is more clearly visible. It is seen to consist of a coating material with particulate matter embedded therein. It will be appreciated that some of the grains 19 of the particulate matter project beyond the surface of the coating 12 while other grains, such as the grains 20, are completely encapsulated by the coating. The grains 1'19 and 20 are part of a contact adhesion enhancement system included in the coating l2. It will be appreciated that the surface of the coating 12 is substantially roughened or contoured as compared to what it would be were the contouring particles 19 and absent. It has been found that contacts applied by the conventional methods such as by application of silver paste or by metallic flame spraying do closely conform to surface irregularities. Thus, it will be appreciated that the contact 13 will adhere more tenaciously to the irregular surface than to a smooth surface. The inclusion of the particulate matter ll9, 20 as described beiow has been found to increase the surface irrc ularity by at least one order of magnitude. Consequently, a substantial enhancement of contact adhesion is obtained.

In order to provide good passivating qualities and to be fully compatible with the pellet 11, the coating 12 preferably comprises at least some of the preselected additives. For example, it has been found that an effective passivating coating can be provided utilizing the reaction product that is discussed in my aforementioned co-pending U.S. application entitled Low Voltage Varistor and Process for Making," filed concurrently herewith. Briefly, the reaction product described therein is formed by thoroughly mixing the preselected additives in the absence of the metal oxide and then prereacting the additives. For example, the additives can be heated and cooled and thus fused into a solid body. This body is then ground to form the reaction product. The metal oxide is mixed with the reaction product and pressed and sintered in the conventional manner.

As specifically described in my previously referenced do-pending U.S. application, a varistor with excellent electrical characteristics can be fabricated from 98 mole percent zinc oxide, 0.5 mole percent bismuth oxide, 0.5 mole percent cobalt oxide, 0.5 mole percent manganese oxide, and 0.5 mole percent titanium oxide.

In accordance with the method disclosed in my 00- pending application, the oxides of bismuth, cobalt, manganese and titanium are thoroughly mixed in equal molar amounts, then heated and cooled to form a crystalline solid body. The crystalline solid body is ground to form the reaction product. Zinc oxide and the reaction product are thoroughly mixed and a portion of the mixture is pressed and sintered to form the varistor body ll 1.

For the specific formulation of a varistor described above, effective passivating coatings can be produced from any of the following base materials:

I. 16 grns. Reaction Product 2. 10.5 grns. Reaction Froduet 5.7 grns. Bi O 3. 14.5 grns. Reaction Product 2.6 grns. H BO 4. 10.5 gms. Reaction Product 5.7 gms. Bi O l.9

gms. H

5. 10.5 gms. Reaction Product 5.7 gms. Bi- O 1.9

gms. H BO 3.6 gms. Sb O 6. 50 gms. Reaction Product 30 gms. Bi O 15 gms. H BO 7 gms. SiO

With respect to the above formulas, it will be appreciated that the sixth formula can be used in medium or high voltage varistors, but it may be desired to avoid the sixth formula, which includes silicon, in the manufacture of low voltage varistors inasmuch as silicon increases the voltage rating of a varistor. Furthermore, it will be appreciated that each of the above formulations is free of silver and other monovalent ions that would tend to diffuse rapidly through the varistor body 11 during the coating heating step to be described subsequently. Such a diffusion of monovalent ions could adversely affect the stability of the varistor l and affect the final electrical properties of the device in a manner difficult to predict. Thus, it is beneficial to omit silver and other monovalent ions from the varistor formulation for reasons more fully discussed in a co-pending US. application Ser. No. 401,323 entitled Silver Free Varistor Passivating Coating, filed by S. l. Gabrail concurrently herewith and assigned to the same assignee as this application.

The compounds of bismuth, boron, antimony, and silicon, where included, are carriers and serve to ultimately enhance the adhesion of the particulate matter 19 and 20 to the body 11.

In order to apply the passivating coating, any of the above formulas is selected and mixed with a vehicle to facilitate handling. A vehicle that has been found effective for the above formulations is composed of l35 grams of n-butyl acetate, 20 grams of ethyl cellulose and 15 grams of Butyl Carbitol.

Application of the coating is as follows. The coating, which when mixed with the vehicle is fluid. is poured over the sintered bodies 11 or the bodies ll are dipped in the coating. The vehicle is then dried at a relatively low temperature. The devices are then heated to a temperature in the range of 600 to l,0O0C for a preselected time. The temperature and time selected should insure that the carrier is fused and thus enhances the adhesion of the particulate matter 19 and 20 to the body 11. However, the particulate matter 19 and 20 should not melt so that a body 11 with the coating 12 and the irregular surface depicted in F l6. 2 is provided. Following the heating step, the metal contacts 13 and 14 are applied by conventional techniques such as the application of silver paste or flame spraying. As mentioned previously, the contacts 13 and 14 will closely conform to the irregular surface of the body 11 and thus adhere tightly thereto.

It will be appreciated that if it is desired for any reason, the carrier can be utilized with different particulate matter. For example, other suitable particulate matter is grains of aluminum oxide.

In summary it will be appreciated that there has been provided a varistor with a passivating coating that reduces leakage current, improves device stability and enhances contact adhesion. Tests have shown that contact adhesion is improved by at least one order of magnitude and leakage current is reduced by several orders of magnitude. Thus, an extremely desirable varistor and method of manufacture have been disclosed.

Many modifications and variations of the subject invention will be apparent to those skilled in the art. For example, other carriers and vehicles can be used. In addition, it will be appreciated that when the aforementioned first formula is used, only reaction product and vehicle is put on the body. Thus when using the first formula, the coating is the reaction product only. F urthermore, it is realized that when certain carriers are employed, they are substantially entirely absorbed into the outer portion of the varistor body but do aid in the adhesion of the particulate matter, which is still embedded therein, although only partially. Consequently, the true scope of the invention is as defined by the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A varistor comprising a body portion defining at least two major surface areas, said body portion having a coating thereon, said coating comprising varistor body surface contour means, and wherein said varistor further comprises metallic contacts overlying said coating on at least a portion of said two major surface areas.

2. A varistor according to claim I wherein said surface contour means comprises particulate matter embedded in said coating.

3. A varistor according to claim 2 wherein said coating is a passivating coating.

4. A varistor according to claim 2 wherein said coating is free of monovalent ions.

5. A varistor according to claim 2 wherein said coating is free of silver.

6. A varistor according to claim 2 wherein said body portion consists essentially of a metal oxide and a reaction product formed by heating together a plurality of preselected additives, and wherein said particulate matter comprises said reaction product.

7. A varistor according to claim 6 wherein said metal oxide is zinc oxide and said preselected additives comprise the oxides of bismuth, cobalt, titanium and manganese.

8. A varistor according to claim 2 wherein said coating comprises at least one member of the group consisting of compounds of bismuth, boron, silicon and anti- 

1. A VARISTOR COMPRISING A BODY PORTION DRFINING AT LEAST TWO MAJOR SURFACE AREAS, SAID BODY PORTION HAVING A COATING THEREON, AID COATING COMPRISING VARISTOR BODY SURFACE CONTOUR MEANS, AND WHEREIN SAID VARISTOR FURTHER COMPRISES METALLIC CONTACTS OVELAYING SAID COATING ON AT LAST A PORTION OF SAID TWO MAJOR SURFACE AREAS.
 2. A VARISTOR ACCORDING TO CLAIM 1 WHEREIN SAID SURFACE CONTOUR MEANS COMPRISES PARTICULATE MATTER EMBEDDED IN SAID COATING.
 3. A VARISTOR ACCORDING TO CLAIM 2 WHEREIN SAID COATING IS A PASSIVATING COATING.
 4. A VARISTOR ACCORDING TO CLAIM 2 WHEREIN SAID COATING IS FREE OF MONOVALENT IONS.
 5. A VARISTOR ACCORDING TO CLAIM 2 WHEREIN SAID COATING IS FREE OF SILVER.
 6. A varistor according to claim 2 wherein said body portion consists essentially of a metal oxide and a reaction product formed by heating together a plurality of preselected additives, and wherein said particulate matter comprises said reaction product.
 7. A varistor according to claim 6 wherein said metal oxide is zinc oxide and said preselected addiTives comprise the oxides of bismuth, cobalt, titanium and manganese.
 8. A varistor according to claim 2 wherein said coating comprises at least one member of the group consisting of compounds of bismuth, boron, silicon and antimony. 